1. Field of the Invention
The present invention relates generally to portable, battery powered devices and more particularly to a battery management system for such portable devices.
2. Description of Related Art
The proliferation of portable devices such as laptop computers, cordless phones, cellular phones, camcorders, and GPS receivers, to name but a few, has caused a correlated increase in the demand for battery pack performance. The consumer demand is constantly increasing for battery packs with higher capacity batteries, longer shelf life, smaller geometries, more usage time between charges, and more features--all at lower cost.
The foregoing demands have led the industry to use batteries of varying chemistries such as, for example, Nickel Cadmium (NiCd) batteries ("Nicads"), Lithium-Ion (Li-Ion) batteries ("Lithium cells"), and NiMH batteries. There is a need, therefore, for a battery management system that is suitable for use with different battery chemistries.
Battery management is of greater concern with some battery chemistries that with others. With Nicads, for example, repeatedly recharging the cells without fully depleting their energy storage will result in undesirable battery "memory" which reduces the capacity of the cells. It may be desirable, therefore, to provide a "fuel gauge" feature which indicates the available energy left based, for example, by estimating energy used since the last full charge. With Lithiums, on the other hand, it is important to protect them from overcurrent or overtemperature conditions because either may result in damage to the battery or worse, in an explosion causing harm to the batter powered components or the user.
A problem with many of the known battery management systems is that they require too many sensing and safety components (at relatively high component manufacturing and assembly cost) and they require that such components be placed in-line, or in series, with the batteries and the battery powered components (at relatively high energy cost where such components have internal resistances).
FIG. 1, for example, illustrates a conventional circuit involving a battery pack 10 that delivers power to battery powered components 70 via. The battery pack 10 contains one or more cells 20, as shown, and typically delivers power to the components 70 via terminals 22, 24 as part of a separable package. As is well known, any power loss between the battery 20 and the battery powered components 70 is simply wasted as heat. This type of loss is often called an IR loss in that it relates to a current I flowing through a resistance R. It is important, of course, to minimize this loss. This goal reduced power loss, however, is often in conflict with the need to provide safety or current sensing components.
FIG. 2 shows an example of a conventional safety component. In the case, the battery pack 10 includes a polyswitch 40--a well known safety device that is essentially a resettable fuse. The polyswitch is normally closed. It opens, however, in the event of overcurrent or overtemperature conditions so as to prevent damage to the battery 20 due, for example, to a short condition. Unfortunately, however, the polyswitch 40 has a characteristic resistance R.sub.ps even when it is closed. As explained above, the current I being drawn through the resistance R.sub.ps is simply wasted as heat.
FIG. 3 shows an example of a conventional current sensing component comprising a current sensing resistor 30 which has a small resistance value R.sub.sense. The voltage which develops across the current sensing resistor 30, in compliance with Ohm's Law, is proportional to the current I flowing through the current sensing resistor 30. So long as R.sub.sense is accurately known, therefore, the voltage V can be measured and I.sub.sense can be derived from the equation I.sub.sense =v/R.sub.sense. In FIG. 3, the induced voltage V is figuratively shown as driving a voltage gauge, but as explained below somewhat with respect to FIG. 4, is normally provided to appropriate circuitry for performing a desired function. Again however, as explained above, the current I being drawn through an inline resistance such as the current sense resistance R.sub.sense is simply wasted as heat.
FIG. 4 is a schematic diagram of a battery pack 10 which incorporates the components of FIGS. 2 and 3 and more fully illustrates the use of the voltage V developed across the current sense resistance R.sub.sense. As shown, the voltage V.sub.sense is delivered to an amplifier and polarity detector 32, is converted to a digital value by an A/D converter, and is passed to a CPU 36 which controls a safety switch 50 based on the current I.sub.sense. In the conventional system, the safety switch 50 is normally closed, and the CPU 36 causes it to open and thereby protect the battery 20 if the current I.sub.sense passes a threshold level that is considered too high. The CPU 36 may also use perform other battery pack functions with the current I.sub.sense, using it, for example, to derive a value for driving a "fuel gauge" 38 which indicates the available battery life. In either event, the battery pack of FIG. 4 suffers from unnecessary energy loss and high component cost associated with the fact that various resistances R.sub.sense and R.sub.ps are inline with the battery powered components 70.
There remains a need, therefore, for a battery management system which effectively protects the battery from damage with reduced component cost and reduced energy loss.